Attention is directed to a master's thesis by one of the inventors herein, J. E. Martini, entitled "The Production and Analysis of Microcellular Foam" (M.I.T., August, 1981) and a master's thesis by another co-inventor herein, F. A. Waldman, entitled "The Processing of Microcellular Foam" (M.I.T., 1982), both of these theses are incorporated herein by reference.
Conventionally foams are produced by using either chemical or physical blowing agents. Chemical blowing agents are generally low molecular weight organic compounds which decompose at a critical temperature and release a gas (or gases), for example, nitrogen, carbon dioxide, or carbon monoxide. See, for example, R. Lacallade, Plastics Engineering, Vol. 32, June 1976, pp. 40-42 for a summary of various types of chemical blowing agents commercially available. Physical blowing agents are gases which are introduced as a component of the polymer charge or under pressure into the molten polymer during processing. See, for example, U.S. Pat. No. 3,796,779 issued to Greenberg on Mar. 12, 1974, where a method of injecting gas into a flowing stream of molten plastic is disclosed.
Most conventional techniques can only produce foams with large cells on the order of 100 microns or larger and with densities about 20-90 percent of the parent material. Fracturing of these large cell foam materials results in low strengths, typically much less than by merely the factor of reduced density.
There exists a need for stronger foam materials than presently available and, preferably, foam materials that are lighter so that less material is used in production without sacrificing the mechanical properties such as toughness. Such foams should have more uniformly-sized voids and generally smaller-sized voids, so that fractures are not initiated from these sites and these cells inhibit or terminate cracks without structural failure. Moreover, it would be advantageous to have foamed materials which exhibit higher toughness and/or strength while using less material.